1. Field of the Invention
The present invention is directed to the field of programmable gate arrays. More particularly, the present invention is directed to a scalable multiple level connector tab network for increasing routability and improving speed of signals in a field programmable gate array.
2. Art Background
A field programmable gate array (FPGA) is a cost effective, high density off the shelf integrated logic circuit which can be programmed by the user to perform logic functions. Circuit designers define the desired logic functions and the FPGA is programmed to process the signals accordingly. Depending on logic density requirements and production volumes, FPGAs are superior alternatives in terms of cost and time to market. A typical FPGA essentially consists of an outer ring of I/O blocks surrounding an interior matrix of configurable function generator (CFG) logic blocks. The I/O blocks residing on the periphery of an FPGA are user programmable such that each I/O block can be programmed independently to be an input or an output and can also be tri-statable. Each logic block typically contains CFGs and storage registers. The CFGs are used to perform boolean functions on its input variables.
Interconnect resources occupy the channel between the rows and columns of the matrix of logic blocks and also between the logic blocks and I/O blocks. These interconnect resources provide flexibility to control the interconnection between two or more designated points on the chip. Usually a metal network of lines is oriented horizontally and vertically in rows and columns between the logic blocks. Programmable switches connect inputs and outputs of the logic blocks and I/O blocks to these metal lines. Cross point switches and interchanges at the intersection of the rows and columns are used to switch signals from one line to another. Often long lines are used to run the entire length and/or breadth or the chip in order to provide point to point connectivity. The functions of the I/O logic blocks and their respective interconnections are all programmable. Typically, these functions are controlled by a configuration program stored in an on-chip or separate memory.
As technology has become more and more sophisticated so has the functionality of FPGAs. The number of CFGs in an array has increased providing for more complex logic functions. It follows that the number of interconnection resources also has increased. Competing with the increased number of CFGs and interconnecting resources is the need to keep the chip as small as possible. One way to minimize the amount of real estate on the chip required is to minimize the routing resources while maintaining a certain level of interconnectivity. Therefore, it can be seen that as the functionality implemented on the chip increases, the interconnection resources required to connect a large number of signals can be quickly exhausted. As a consequence, most CFGs are either left unused due to inaccessibility or the CFGs are used simply to interconnect wires instead of performing certain logic functions. This can result in unnecessarily long routing delays and low logic utilization. The alternative is to provide more routing resources which can increase the chip die size dramatically.
An improved field programmable gate array (FPGA) is provided which includes tab network connectors for interfacing groups of logic cells with lower levels of interconnect and for interfacing lower levels of interconnect with higher levels of interconnect. In one embodiment, the connector is used to interface a group of elements or configurable function generators (CFGs), including storage elements, to certain levels of a hierarchical routing network. Each group or cluster of a logic block is formed of multiple CFGs programmably coupled to a set of bidirectional input/output lines. In the present embodiment an innovative cluster architecture is utilized which provides fine granularity without a significant increase in logic elements. The bidirectional input/output line is coupled to the connector. The connector includes a connector tab line coupled to the bidirectional input/output line through a programmable switch. The connector tab line is also coupled to the connector and bidirectional input/output line of an adjacent block. Frequently, signal routings occur between adjacent blocks, and in the prior art valuable routing lines which interconnect to higher levels of the routing hierarchy were used. In the improved FPGA of the present invention, a signal from a logic block can be directly routed to an adjacent logic block without utilizing the network of routing lines. This frees up the valuable routing lines to perform longer, non-adjacent block routings, and therefore the space required for non adjacent routing can be optimized. An additional, significant advantage is the minimizing of blockage caused by signal routings as each bidirectional input/output line is selectively coupled through two block connector tab networks to the routing hierarchy.
Also coupled to the bidirectional input/output line is a plurality of bidirectional switches that are programmable to permit a signal originating from the bidirectional input/output line to couple to one or more of a plurality of levels of hierarchical routing lines. A first programmable driver and second programmable driver are programmably coupled between the bidirectional input/output line and the plurality of switches. The first driver drives the signal received from the logic cells via the bidirectional input/output line out to one or more routing lines of the hierarchy of routing lines through determined programmable switches. The second driver takes a signal received from a routing line of the hierarchy of routing lines through a programmable switch to the bidirectional input/output line. Thus, a flexible, programmable connector is provided. Furthermore, the connector can be programmed to provide a xe2x80x9cfan outxe2x80x9d capability in which the connector drives multiple routing lines without incurring significant additional signal delay and without using multiple tab connector networks.
In another embodiment, the tab connector network can also be used to route a lower level routing line to a higher level routing line. This is particularly desirable in order to meet the needs for driving a signal along longer routing lines without requiring all signal drivers be sufficiently large to drive a signal along the longest routing line. In particular, routing tabs lines are provided that span distances equivalent to a third level of the routing hierarchy. A tab network is coupled to each routing tab line to programmably connect each block through the tab line to one of a plurality of higher level routing lines. The connector includes programmable bidirectional drivers to drive the signal along the longer higher level routing lines of the routing hierarchy.
These connector networks enable a flexible routing scheme to be implemented in which the routing lines at each level are divided into sets. For example, one set can be accessible by a first set of logic elements or CFGs and a second set accessible by a second set of logic elements or CFGs. The first set of routing lines are accessible to the second set of logic elements or CFGs via the corresponding connector networks for the second set of logic elements or CFGs. Similarly, the second set of logic elements or CFGs can access the first set of routing lines via the connector networks for the first set of logic elements or CFGs. It follows that the first set of CFGs and second set of CFGs can access both sets of routing lines thereby minimizing the likelihood of routing blockage of the signal.
Furthermore, a turn matrix is preferably included to cause the signal located on one routing line to transfer to a routing line in a different orientation. For example, a turn element of a turn matrix enables the signal to transfer between a horizontal and vertical routing line. As turn matrices require a significant amount of space on the chip, the connector networks can be utilized to provide sufficient connectivity, especially for the most commonly occurred two segments diagonal connect while minimizes the real estate for turn matrices. In particular, the connector networks enable the device to implement partial turn matrices, wherein up to half the number of turn elements are eliminated to save space on the chip.
In addition, this innovative routing hierarchy consisting of the multiple levels of routing lines, connector tab networks and turn matrices, permits an innovative, space saving floor plan to be utilized in an integrated circuit implementation, and is particularly efficient when SRAM is used as the configuration bits. This floor plan is a scalable block architecture in which each block connector tab networks of a 2xc3x972 block grouping is arranged as a mirror image along the adjacent axis relative to each other. Furthermore, the bidirectional input/output lines provided as the input/output means for each block are oriented only in two directions (instead of the typical north, south, east and west directions) such that the block connector tab networks for adjacent blocks face each other in orientation. This orientation and arrangement permits blocks to share routing resources. This reduces the routing segments requirement. In addition, this arrangement enables either a 2xc3x972 block or a 4xc3x974 block grouping to be scalable.
The innovative floor plan also makes efficient use of die space with little layout dead space as the floor plan provides for a plurality of contiguous memory and passgate arrays (which provide the functionality of the bidirectional switches) with small regions of logic for CFGs and drivers of the block connector tab networks. Therefore, the gaps typically incurred due to a mixture of memory and logic are avoided. Intra-cluster routing lines and bidirectional routing lines are intermixed and overlayed on different layers of the chip together with memory and passgate arrays to provide connections to higher level routing lines and connections between CFGs in the block.